In astronomy, a Galaxy is generally a gravitationally bound large collection of matter, such as stars and planetary systems, nebulae, clouds of dust and other objects.
Most galaxies are classified according to their form in different groups and subgroups of the so-called Hubble classification. This classification was established by Edwin Hubble and is with some extensions still in use.
The different types are:
- Elliptical galaxies contain only old red stars and almost no gas, so their star formation rate is nearly zero. Elliptical galaxies are classified according to their numerical eccentricity in the classes E0 (circular) to E7 (highly elliptical). The number following the E indicates the first decimal place of eccentricity, for example, a Galaxy Class E7 has the eccentricity 0.7. The absolute magnitudes of Elliptical galaxies span a very wide range. The brightest galaxies are mostly elliptical galaxies and are in this case probably formed by the merger of several small to medium-sized galaxies. Elliptical galaxies are often found in large clusters of galaxies.
- Lenticular (lens-shaped) galaxies belong to the class S0. They have a core which corresponds to the spiral galaxies, but their galactic disk contains no spiral arms, but is approximately uniformly bright (Example: M 102).
- Spiral galaxies consist of a central core in which the stars are relatively close together, and from there going off spiral arms that come directly from the core. As the bulge is similar to an elliptical galaxy and shows no more star formation, the in the disc existing gas and dust enbale the star formation in the spiral arms. Therefore, the spiral arms appear on images mostly blue and the nucleus usually reddish. The spiral arms are further divided into the classes Sa, Sb and Sc. Galaxy of type Sa have a very distinct core and tightly-wound spiral arms (example: Sombrero Nebula M 104). The type Sc has a relative weak galactic core, extremely loosely wound spiral arms and thereby sometimes almost the shape of a self tortuous “S” (example: the Triangulum Galaxy M 33). The lenticular galaxies together with the Sa, Sb and Sc re also called disk galaxies. An obsolete alternative name is spiral nebula.
- Barred spiral galaxy have a long bar, starting from the center to the end of which connect the spiral arms (example: M 109). Like the spiral galaxies they are divided by increasing severity of the core and the opening of their spiral arms in the classes SBa, SBb and SBc.
- Irregular galaxies have neither spiral arms nor elliptical shape. They are in middle light-weaker than elliptical and spiral galaxies. This group includes usually dwarf galaxies.
It is estimated that the elliptical, spiral or irregular shape of a galaxy depends from the rotation in the gas cloud from which it is formed, and its rate of star formation. In the distant future, when all or most of the gas will be exhausted, the star formation in the galaxies will stop. It is believed that finally most of the matter of a galaxy collapses to a black hole.